CELLS IN DIVISION 



these differences. The Hving pollen mother cell has been studied by 

 several workers, though the complete meiotic cycle has yet to be 

 followed in a single preparation of isolated cells. Gregory^^ attempted 

 to cultivate excised anthers of several species in a nutrient solution; and 

 found that the early phases of meiosis never progressed in his material, 

 but that in Lilium anthers, pollen mother cells at diplotene would 

 continue the cycle to the formation of quartettes. Stern^^ found that 

 those of Trillium cultivated in sucrose lost their cell walls and fused 

 together. In Tradescantia, meiosis proceeded comparatively rapidly, 

 and Shimakura^* was able to observe and photograph the remaining 

 stages in pollen mother cells mounted at metaphase during the subse- 

 quent eight hours or so. Successful cultivation of excised whole anthers 

 has recently been achieved by Taylor. ^^ 



The haploid mitosis within the pollen tube has not yet been followed 

 in the living state, although the necessary technique for the artificial 

 germination of the pollen grain is available (Darlington and La 

 Cour36). 



Protozoa — In Belar's^'^ great monograph on the Protozoan nucleus 

 his Table I lists investigations on living Protozoa in mitosis from which 

 it can be seen that several such studies date from the nineteenth century. 

 Of these, the most complete was that of Hertwig,^^ who in 1884 

 followed nuclear division in Actinosphaerium. Before turning his attention 

 to the Metazoan cell in division, Belar himself studied the mitosis of 

 several Protozoa in both living and fixed individuals and published 

 papers on Chilomastix and Bodo (Belar^^) and on Actinophrys (Belar*°). 

 In the latter paper the drawings of the living nucleus during mitosis 

 are particularly fine. 



Among the Amoebidae, little is seen of the nucleus during fission in 

 either Amoeba proteus (Chalkley and Daniel ;^^ Dawson et alii^^) or 

 A. dubia (Dawson et alii^^), but Comandon and de Fonbrune^* have 

 found that in a large Acanthamoeba the nuclear detail is exquisitely 

 visible throughout fission. Their paper contains no illustrations, how- 

 ever, and is limited to a description of the admirable film which they 

 have made of mitosis in this organism. These authors have generously 

 provided a series of still photographs from their film for reproduction 

 in this book and these are to be found in Plate VI. 



The resting nucleus of Acanthamoeba is circular, with a diameter of 

 i0|jl; a central nucleolus occupies two-thirds of the volume of the whole 

 nucleus. Close to the nuclear membrane on one side is a centrosomal 

 body from which a number of definite rays originate, in such a manner 

 that Comandon and de Fonbrune describe it as a 'figure en arraignee'. 

 A full hour before mitosis is due to begin, an individual which will 

 divide can be recognized because there is now a 'figure en arraignee' 

 at either end of the nucleus, which has become elliptical. As prophase 



75 



